The limits to switching and modulation speed of semiconductor lasers are fundamental bottlenecks for information processing and transmission in optical communication systems. These limits come from the relatively slow carrier recombination lifetime in the III-V semiconductors used for optical applications. Though progress in pushing this limit in the past has been made, these improvements cannot meet the demand for higher speed in the long run. A new paradigm has to be explored to maintain the momentum of technology development.
The ability to steer or switch the propagation direction of a laser beam in a controllable way is very important for many applications, and especially for optical interconnect networks. Beam scanning and steering in edge emitting lasers have been realized using thermal control (See Y. Sun, C. G. Fanning, S. A. Biellak, and A. E. Siegman, IEEE Photonics Technol. Lett. 7,26(1995)) and spatial phase controlling techniques (See J. P. Hohim, D. C. Craft, G. A. Vawter, and D. R. Myers, Appl. Phys. Lett. 58, 2886 (1991)). For optical interconnect applications, all the well-known advantageous attributes of vertical_cavity surface emitting lasers (VCSELs) make them especially appealing elements. However, full advantage cannot be taken of compact two-dimensional (2D) VCSEL arrays if bulky external passive optical elements are used for routing and switching. For this reason, routers integrated together with VCSELs that can be controlled electronically are especially important to reduce the overall volume of an interconnect network. See L. Fan, M. C. Wu, and P. Gradzinski, Electron. Lett. 31, 729 (1995) and L. Fan, M. C. Wu, H. C. Lee, and P. Grodzinski, IEEE Photonics Technol. Lett. 9, 505(1997).
Recently it has been demonstrated that, by introducing a phase-shifter in part of the VCSEL cross-section, beam switching of up to 2 gigahertz can be achieved. Another more conventional approach to VCSEL beam manipulation is to use VCSEL arrays. Indeed, VCSEL arrays of various kinds have been quite extensively researched for tailoring and engineering near and far field patterns. See M. Orenstein, and T. Fishman, “Supermodes of Hermite Tapered Arrays of Vertical-Cavity Semiconductor Lasers,” IEEE Jour. Quantum Electron. 35, 1062-1066 (1999). See D. Natan, M. Margalit, and M. Orenstein, “Localization Immunity And Coherence Of Extended Two-Dimensional Semiconductor Vertical Cavity-Locked Laser Arrays,” Jour. Opt. Soc. Am. B 14, 1501-1504, (1997). See T. Fishman, and M. Orenstein, “Cyclic Vertical Cavity Semiconductor Laser Arrays With Odd Numbers Of Elements: Lasing Modes And Symmetry Breaking,” Opt. Lett. 21, 600-602, (1996).
Optical communications in terrestrial environments and in space require very high frequency transmitted signals, of the order of 40 GHz and higher, in part because of the uncertain and changing transmission environments. What is needed is a robust optical communication system that will provide these frequencies, that provides substantial discrimination between different symbols, that permits switching times of the order of 0.25 picoseconds, and that permits the use of two or more distinct symbols.